Radiotherapy and radiosurgery are non-invasive radiation treatment options widely used to treat patients with a variety of tumors such as brain tumor, lung tumor, and liver tumor. Fiducial tracking is one among a variety of conventional tracking methods utilized in performing radiation treatments.
Electromagnetic tracking systems are used to track the positions of fiducials in near real time (e.g., 10 Hz). In one type of system (i.e., Calypso® 4D Localization System, available from Calypso, of Seattle, Wash.), an array of four transmitter coils spread out in space induces a resonance in a fiducial or transponder coil system. When the magnetic field is switched off, the transponder signal during relaxation is sensed by an array of receiver coils and used to establish the position of the transponder. In the Calypso 4D Localization System, the transmitter and receiver coils are embedded in an array wired to a common device, and the fiducials or transponders are wireless.
A tracking system may provide the positions of fiducials or transponders that are implanted in a patient before acquisition of a treatment planning computed tomography (CT) scan, for example for radiation therapy. The fiducial positions are known relative to the transmitter coils or electromagnetic localization array. If the transmitter coil or array position is known in the treatment room, the fiducial positions relative to the treatment room isocenter (treatment room coordinate system origin) can be obtained. The fiducial positions can then be used to guide treatment, for example, by directing a radiation beam at a target tracked by reference to the fiducial positions.
In such a tracking system, inappropriate placement of the localization array may cause problems such as susceptibility to interference in the array or physical obstruction of other elements of the treatment system.
Another problem with electromagnetic tracking systems is that the accuracy of the position and orientation information is affected by changes in the magnetic field other than those created by the transmitter coils. Perturbations in the magnetic field can be caused by the presence of metal (for example, in radiation therapy, a gantry or robotic manipulator and a linear accelerator) or other conductive material (for example, a treatment table top made out of conductive material). Thus the position information reported by an electromagnetic tracking system in practical use, for example, tracking a target region inside a patient during radiation therapy, can have a higher error than a system specification determined in a carefully controlled laboratory setting.